Greek coast guard vessels were on Wednesday night searching for an Iranian tanker that disappeared after being refused a request to refuel off the island of Syros as it was suspected that the vessel was carrying Iranian oil in contravention of a European Union ban.

The Tanzanian-flagged Baikal, part of top Iranian tanker operator NITC’s fleet, approached Syros on Tuesday in order to refuel but officials from local company Sekavin refused after they, along with customs officers, checked the documents accompanying the shipment of 146,000 tons of crude oil. The documents did not detail who would be the recipient of the fuel.

“We conducted an initial investigation in order not to break the law. In the end, we did not provide them with any fuel because it would be against the rules,” a Sekavin official told Reuters. “Regardless of its flag, the ship represents Iranian interests.”

Sources said that at 8 p.m. on Tuesday, the Baikal set sail from Syros and 17 minutes later, when it was heading toward Myconos, the vessel’s Automatic Identification System was switched off.

Customs officials on Syros notified the Financial Crimes Squad (SDOE), which does not have a vessel that can be used to track down suspects. As a result, the coast guard was called in. Authorities suspect that the Baikal was delivering the oil to a customer in Greece or the Balkans in contravention of an EU embargo.

The block on Iranian oil exports to Europe and a ban on European ship insurance are among the tougher measures imposed on the Islamic Republic over its nuclear program, aiming to make Western companies halt trade with the OPEC member country.